A first type of heat exchanger, a very widespread type for industrial use, is that of the so-called “flooded” evaporators.
As well known for a person skilled in the art, this type of exchanger provides a skirt acting as outer casing, inside which one or more tube bundles are housed, wherein a first operating fluid flows, in particular a so-called “hot” fluid. Inside the skirt, then, over the free surface, a so-called “cold” second operating fluid, that is a refrigerating fluid, is fed. The latter laps against the tube bundle(s) with the purpose of the heat exchange with the first fluid, it subtracts heat to the latter and evaporates by flowing towards a vapour-sucking orifice placed on the top.
The second fluid, at the end of the stage of thermal exchange with the first fluid and therefore at the top of the skirt of the exchanger, should result wholly vaporized. However, a drawback which often is met is that in the second operating fluid liquid particles remain which can damage the components downwards the exchanger or however determining an operation under not nominal conditions thereof.
In order to avoid or limit said drawback, the extension of the free surface of the refrigerant inside the skirt is made very wide. This is obtained by conferring the skirt a strongly widened, and in particular horizontally elongated shape. The extension of the skirt is strongly prevalent in a horizontal direction orthogonal to the flow direction of the second fluid inside the skirt itself and parallel to the extension direction of the tubes inside thereof the first “hot” operating fluid flows. In particular, the section area of the skirt on the horizontal plane is highly prevalent with respect to the one of the vertical section enveloping the tube bundle involved by the first operating fluid, the relationship between the two areas being higher than 2.5.
Still to obviate said drawback, the free surface is kept quite “low” with respect to the top of the skirt wherein the vapour-sucking orifice is placed. In this way, the “ascending” speed of the vapour from the free surface towards the sucking orifice is very low and consequently the dragging of liquid drops during the ascent is limited.
However, said widened shape of the exchanger generally makes it very bulky. Furthermore, the huge cross extension of the free surface involves a huge consumption of refrigerant fluid which, as it is known, has very high costs as well as an important environment impact.
Furthermore, still in order to avoid the above-mentioned drawback, an auxiliary unit for overheating the second operating fluid, or a system for filtering the dragged drops of liquid or even a system which makes it difficult the passage of refrigerant drops downwards the primary tube bundle with respect to the flow of the second operating fluid. Even these expedients involve an increase in the overall dimensions and, of course, in the costs.
Another very widespread type of heat exchanger for industrial use is that of the so-called “falling-film” evaporators.
As well known for a person skilled in the art, even the type of “falling-film” evaporator provides a skirt acting as outer casing, inside thereof one or more tube bundles are housed wherein a first operating fluid flows, in particular a so-called “hot” fluid. In the falling-film configuration of “pure” type, a second so-called “cold” operating fluid—that is a refrigerating fluid—is fed inside the skirt only through a distribution system with nozzles preferably placed above the tube bundles mentioned above. The liquid phase of such second fluid deposits onto the outer surface of the tubes of the row immediately below the distribution system, in this way by exchanging heat with the primary fluid and by evaporating partially. The remaining liquid portion “falls” by gravity onto the rows of the lower tubes, by distributing effectively even thereon, by forming a liquid “film” and thus by triggering an evaporation process with high efficiency of thermal exchange.
In the type of evaporators with falling film of hybrid type, a tube portion of the tube bundle arranged in the lower portion of the skirt is wholly dipped in the liquid refrigerant, by operating in reality like the type of the “flooded” evaporators, whereas the upper portion of the tube bundle operates like the just described pure type of the “falling-film” evaporators.
Even in this second type of evaporators, the second fluid, at the end of the stage of thermal exchange with the first fluid and therefore at the top of the skirt of the exchanger, should result wholly vaporized. However, even in this case in the second operating fluid liquid particles remain which can damage the components downwards of the exchanger or however determine an operation under not nominal conditions thereof. In the herein considered type of evaporator this drawback is particularly difficult to be avoided as the refrigerant outgoing from the distribution system is in counter-flow with respect to the mass of the ascending vapour produced by the evaporation of the refrigerant on the tubes and directed towards the sucking orifice of the exchanger. The mass flows of these opposed flows are approximately equal and typically equal to the nominal rate of the refrigerating machine thereto the evaporator belongs.
To obviate such drawback, a first solution consists in using a separator of liquid/vapour placed on the refrigerant circuit, downwards the throttling valve, upwards an inlet/recirculation of the refrigerant in the distribution system which feds the evaporator. The separated vapour is conveyed on the sucking line of a compressor or however it does not come in contact with the tube bundle of the evaporator, whereas the accumulated liquid is brought to feed the evaporator by means of the distribution system. In this way a smaller mass flow of the refrigerant flowing into the evaporator is obtained, and therefore fewer dragging problems and a consequent better distribution of liquid even on the lower rows of the tube bundle, as the distribution thereof by gravity is less disturbed by the flow of the ascending vapour.
A second adopted solution is that of using a so-called “in-line” configuration of the tube bundle, wherein the tubes are arranged in horizontal rows and vertically aligned. In such way, the exceeding liquid falling by gravity finds thereunder an aligned whole column of tubes and, at the same time, the ascending vapour finds extension passage “preferential lanes” equal to the distance between two columns of adjacent tubes. In such way, the liquid-dragging effect and the disturbing effect of the distribution of the latter on the tubes are reduced. However, at the upper rows of the tubes (that is near the distributor, wherein the opposed mass flows are high) the problem of the liquid dragging is not solved in a satisfying way.
Another adopted solution is to use a hood wrapping on the top and on the side the tube bundle and prevents the produced vapour to flow in counter-flow with respect to the liquid refrigerant in the fall by gravity on the rows of tubes. In particular, in such solution the distributor is generally placed inside the hood—on the top of the tube bundle—and the configuration is so that the distributed liquid and the produced vapour both follow in the same direction, from the top to the bottom, as far as the vapour outgoes from the hood through suitable side openings and it can proceed through suitable channels ascending towards the sucking orifice. In such configuration, generally a lower portion of the tube bundle is left to operate wholly dipped in the liquid refrigerant, so as to receive and to make to evaporate the liquid not evaporated on the upper tubes. However, even this solution involves an increase in the involved volumes.
In brief, the known evaporators considered sofar request huge volumes on the refrigerant side, have huge overall plan dimensions due to the development of the skirt on the horizontal plane and generally they require additional components to solve the problem of dragging the liquid to the sucking orifice of the evaporated refrigerant.